Due to good resistance to solvents, excellent mechanical strength, and electrically insulating properties, etc., epoxy resin is widely used. For example, epoxy resins are often applied to coating materials, electrically insulating materials, printed circuit laminated boards and electronic packaging materials, construction and building materials, adhesives, and navigation technology. Epoxy resins, however, can have poor thermal resistance and burn easily, which may set significant restriction on the uses of epoxy resin. Therefore, with development of electronic technology, the industry has sought to improve flame retardant properties and thermal resistance of epoxy resins.
There has been a plurality of techniques available for improving the flame retardant properties of epoxy resins, the most common one of which is to introduce a flame retardant into an epoxy resin compound. Often, a halogen-containing flame retardant is used. Although halogens are effective for retarding flames, they can produce erosive and toxic hydrogen halide gases.
Phosphorus-containing flame retardants have significant advantages of low toxicity, good processing properties, low usage amounts, and good compatibility with resins. During the burning process of the phosphorus-containing flame retardants, on the one hand, polymeric materials are urged to undergo a dehydration reaction by which hydrogen of carbohydrate reacts with oxygen of air to form water so as to reduce an ambient temperature and thereby provide a flame retardant effect. On the other hand, phosphoric acid is decomposed under a high temperature, making polymeric compounds carbonized to form a flame retardant coke layer; moreover, phosphoric acid would be further dehydrated and esterified under the high temperature to form glass-like melted polymeric phosphoric acid that covers surfaces of burning substances and serves as a protective layer for preventing oxygen from entering into non-burning internal portions of polymers and for impeding release of volatile decomposed substances, thereby inhibiting proliferation of flames and achieving the flame retardant effect.
Currently used phosphorus-containing substances can be divided into reactive phosphorus-containing compounds with function groups, and generally non-reactive phosphorus-containing compounds. The non-reactive phosphorus-containing compounds have relatively poor thermal resistance and are not suitably applied to epoxy resin compositions required to be highly thermal resistant. The reactive phosphorus-containing compounds bonded to other molecules can thus have relatively higher thermal stability and thereby become a mainstream of usage.
Among available reactive phosphorus-containing compounds, the most commonly used is a linear phosphorus-containing compound; however, due to an —O—P—O— bond on a main chain thereof, this linear phosphorus-containing compound has poorer thermal resistance than a normal halogen-containing or halogen-free epoxy resin composition. In another aspect, phosphorus-containing flame retardant resin compositions, no matter having linear phosphorus-containing compounds or non-reactive phosphorus-containing compounds, are worse in processing properties than bromine-containing epoxy resin compositions in practical applications. Therefore, it is deemed hard to enhance both the flame retardant properties and thermal resistance of the resin compositions.